diff --git a/src/openrct2/rct12/RCT12.cpp b/src/openrct2/rct12/RCT12.cpp
index 762f4035c3..703c70c177 100644
--- a/src/openrct2/rct12/RCT12.cpp
+++ b/src/openrct2/rct12/RCT12.cpp
@@ -1009,7 +1009,7 @@ bool RCT12ResearchItem::IsRandomEndMarker() const
     return rawValue == RCT12_RESEARCHED_ITEMS_END_2;
 }
 
-ObjectEntryIndex RCTEntryIndexToOpenRCT2EntryIndex(RCT12ObjectEntryIndex index)
+ObjectEntryIndex RCTEntryIndexToOpenRCT2EntryIndex(const RCT12ObjectEntryIndex index)
 {
     if (index == RCT12_OBJECT_ENTRY_INDEX_NULL)
         return OBJECT_ENTRY_INDEX_NULL;
@@ -1017,7 +1017,7 @@ ObjectEntryIndex RCTEntryIndexToOpenRCT2EntryIndex(RCT12ObjectEntryIndex index)
     return index;
 }
 
-RCT12ObjectEntryIndex OpenRCT2EntryIndexToRCTEntryIndex(ObjectEntryIndex index)
+RCT12ObjectEntryIndex OpenRCT2EntryIndexToRCTEntryIndex(const ObjectEntryIndex index)
 {
     if (index == OBJECT_ENTRY_INDEX_NULL)
         return RCT12_OBJECT_ENTRY_INDEX_NULL;
diff --git a/src/openrct2/rct12/RCT12.h b/src/openrct2/rct12/RCT12.h
index eeda46ec35..8512426e2f 100644
--- a/src/openrct2/rct12/RCT12.h
+++ b/src/openrct2/rct12/RCT12.h
@@ -820,5 +820,5 @@ assert_struct_size(RCT12ResearchItem, 5);
 
 #pragma pack(pop)
 
-ObjectEntryIndex RCTEntryIndexToOpenRCT2EntryIndex(RCT12ObjectEntryIndex index);
-RCT12ObjectEntryIndex OpenRCT2EntryIndexToRCTEntryIndex(ObjectEntryIndex index);
+ObjectEntryIndex RCTEntryIndexToOpenRCT2EntryIndex(const RCT12ObjectEntryIndex index);
+RCT12ObjectEntryIndex OpenRCT2EntryIndexToRCTEntryIndex(const ObjectEntryIndex index);